vutil.c: Add logic to avoid having to change locales

Prior to this commit, unless the locale was C or POSIX, it changed it
temporarily so as to make sure the decimal point radix character was a
dot.

But on POSIX platforms or C99 compilers, there are easy ways to
determine if the radix is a dot or not, without having to look at the
locale.  This commit changes to take advantage of those mechanisms to
avoid toggling the locale unless it is actually needed to be done.

Only if the compiler is C89 on a non-POSIX box will the previous
existing method be used.

vutil.c

@@ -638,20 +638,43 @@ Perl_upg_version(pTHX_ SV *ver, bool qv)
             /* Otherwise, toggle the locale to one containing a dot radix (if
              * needed) */
             const char * locale_name_on_entry;
+            bool need_to_toggle;
 
             LC_NUMERIC_LOCK(0);    /* Start critical section */
 
-            locale_name_on_entry = setlocale(LC_NUMERIC, NULL);
-            if (   strNE(locale_name_on_entry, "C")
-                && strNE(locale_name_on_entry, "POSIX"))
+#    if defined(I_LANGINFO) && defined(HAS_NL_LANGINFO)
+#      include <langinfo.h>
+
+            /* On POSIX boxes, it is easy to find if the current radix is a
+             * dot. */
+            need_to_toggle = strNE(nl_langinfo(RADIXCHAR), ".");
+
+#    elif defined(HAS_SNPRINTF)
+
+            /* Without nl_langinfo(), we can use snprintf() to see if the
+             * decimal point is a dot. */
             {
-                /* the setlocale() call might free or overwrite the name */
-                locale_name_on_entry = savepv(locale_name_on_entry);
-                setlocale(LC_NUMERIC, "C");
+                char floatbuf[4] = { '\0' };
+
+                /* 1.5 is exactly representable on binary computers */
+                need_to_toggle =
+                    (   3 != snprintf(floatbuf, sizeof(floatbuf), "%.1f", 1.5)
+                     || strNE(floatbuf, "1.5"));
             }
-            else {  /* This value indicates to the restore code that we didn't
-                       change the locale */
-                locale_name_on_entry = NULL;
+
+#    else     /* Fallback for C89 compilers only. Assume just these locales have
+               a dot radix */
+
+            locale_name_on_entry = setlocale(LC_NUMERIC, NULL);
+            need_to_toggle =    strNE(locale_name_on_entry, "C")
+                             && strNE(locale_name_on_entry, "POSIX");
+#    endif
+
+            /* If the radix isn't known to be a dot; toggle to a locale that is
+             * so known */
+            if (need_to_toggle) {
+                locale_name_on_entry = savepv(setlocale(LC_NUMERIC, NULL));
+                setlocale(LC_NUMERIC, "C");
             }
 
 #  endif    /* end of ! USE_POSIX_2008_LOCALE */
@@ -678,7 +701,7 @@ Perl_upg_version(pTHX_ SV *ver, bool qv)
 
             uselocale(locale_obj_on_entry);
 #  else
-            if (locale_name_on_entry) {
+            if (need_to_toggle) {
                 setlocale(LC_NUMERIC, locale_name_on_entry);
                 Safefree(locale_name_on_entry);
             }